This invention relates to a technique for increasing the catalytic activity of crystalline zeolites. In particular, a novel activation process is provided to enhance the alpha value of high-silica zeolites by hydrothermal treatment in contact with an aluminum phosphate activating agent.
Zeolite catalysts have become widely used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, alkylation, dealkylation, transalkylation, isomerization, polymerization, addition, disproportionation and other acid catalyzed reactions may be performed with the aid of these catalysts. Both natural and synthetic zeolites are known to be active for reactions of these kinds.
Recently, synthetic zeolites containing high proportions of silica relative to alumina have been developed and zeolites of this kind have shown themselves to be useful. U.S. Pat. No. 3,702,886 to Argauer et al discloses a class of crystalline aluminosilicates designated ZSM-5 which have highly advantageous properties. U.S. Pat. No. 3,941,871 and its U.S. Pat. No. Re. 29,948 to Dwyer et al disclose crystalline organosilicates which exhibit a structure, as evidenced by X-ray diffraction pattern, similar to that of ZSM-5, but with high ratios of silica relative to alumina. Materials of this kind are stated to exhibit low aging rates and to have low coke making properties when used in hydrocarbon processing.
Various treatments have been proposed in the past for modifying the activity of the zeolites, either by reducing it when too active or by increasing it when insufficient. One such treatment has been steaming and in the past it has generally been used to decrease the activity of the zeolite, as reported in "Fluid Catalytic Cracking with Zeolite Catalysts", Venuto and Habib, Marcel Dekker Inc., N.Y., N.Y. 1979.
The reduction of activity is not, however, necessarily undesirable because it may in certain circumstances be accompanied by an improvement in other characteristics of the zeolite, for example, resistance to aging. This fact has been exploited in certain processes, for example, in the alkylation process described in U.S. Pat. No. 4,016,218, which employs a zeolite catalyst which has been subjected to a prior thermal treatment either in an inert atmosphere or by steaming, to reduce its activity. The deactivation caused by the steam becomes more pronounced at higher temperatures and with longer reaction times.
It has also been found that steaming may in certain instances have beneficial effects upon the catalyst. U.S. Pat. No. 3,257,310, for example, describes a method for preparing a cracking catalyst of high activity and selectivity by steaming a zeolite for at least two hours at a specified temperature. The zeolites described in this patent include natural zeolites such as mordenite and faujasite and synthetic zeolites such as zeolites X, Y and L.
U.S. Pat. Nos. 4,149,960 and 4,150,062 describe the use of water in the feedstock during operation to reduce coking and aging rates. U.S. Pat. No. 3,546,100 describes a method for maintaining the selectivity of a hydrocracking catalyst by restricting the partial pressure of water during the hydrocracking operation.
U.S. Pat. No. 3,493,519 describes a method of producing hydrothermally stable cracking catalysts by calcining zeolite-Y in the presence of steam, a process which was theorized to cause lattice aluminum defects which, after subsequent treatment by base exchange with ammonium salts, chelation and calcination in air produced the desired highly active product.
U.S. Pat. No. 3,493,490 describes a method for restoring the activity to used catalyst by controlled treatment with anionic reagents including water at high temperatures, even with catalysts which had initially been steamed to reduce their level of cracking activity, such as zeolites X and Y.
U.S. Pat. No. 3,758,403 describes a method for cracking hydrocarbon feedstocks using a mixture of zeolites including a ZSM-5 type zeolite and a large pore zeolite such as zeolites X, Y or L or faujasite. The selectivity of the catalyst is said to be improved by treatment with steam which if desired, may be carried out in the cracking unit itself.
Previous work in activation has shown substantial increases in activity for ZSM-5 type zeolites treated with aluminum halides or organo-aluminum complexes at elevated temperatures. Fluoride treatment at various pH levels has also been studied. Another zeolite activation technique employed steaming a high silica (1600:1 SiO.sub.2 :Al.sub.2 O.sub.3) crystalline alumino-silicate acid ZSM-5 in intimate mixture with an activating metal oxide, such as alpha alumina, at high temperature to achieve increased cracking activity.
Although certain zeolites may be described as materials containing silica and alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. More particularly, GeO.sub.2 is an art recognized substitute for SiO.sub.2 and B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, and Ga.sub.2 O.sub.3 are art recognized replacements for Al.sub.2 O.sub.3. Accordingly, the term zeolite as used herein shall connote not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and/or aluminum. On the other hand, the term aluminosilicate zeolite as used herein shall define zeolite materials consisting essentially of silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, as opposed to materials which contain substantial amounts of suitable replacement atoms for such silicon and/or aluminum. Furthermore, descriptions herein referring to zeolites as being boron-containing shall connote that the zeolite contains boron in the framework structure thereof, e.g., in place of at least a portion of the framework aluminum in an aluminosilicate zeolite.